1. Field of the Invention
The present invention relates to the preparation of rare earth ammonium double oxalates and to the production of rare earth oxides therefrom.
This invention especially relates to the preparation of such double oxalates having a well defined morphology and particle size.
2. Description of the Prior Art
The rare earth oxides find numerous applications, particularly in the field of ceramics and electronics, but there currently exists an increasing demand for such products having a controlled particle size.
One of the conventional processes for preparing rare earth oxides which is widely described in the literature, in particular in Paul Pascal's Nouveau Traite de Chimie Minerale, Vol. III, p. 1007 (1959), entails calcining, at a temperature ranging from 500.degree. to 900.degree. C., the rare earth oxalates obtained by precipitation of the corresponding rare earth salts in the form of an aqueous solution by means of oxalic acid. However, a process of this type produces particulates of rare earth oxides having a large particle size.
It has also been described, in JP-A 53/095,911 (Chemical Abstracts, 90, 40940 w), to prepare finely divided rare earth oxides and, more particularly, finely divided yttrium oxide, by calcination of a yttrium ammonium oxalate, which entails providing an aqueous solution of a yttrium salt as the starting material, precipitating the yttrium in the form of its hydroxide by reaction of the aqueous solution of the yttrium salt with a basic aqueous solution such as ammonia, then treating the resultant hydroxide slurry with oxalic acid and finally separating off the precipitate thus obtained, washing it and calcining the precipitate at a temperature of 750.degree. C. According to this JP-A 53/095,911, such process produces finely divided yttrium oxide. The diameter of the particles is said to range from 0.9 to 4.5 .mu.m, the crystals having the shape of platelets with rounded edges.
However, the size of these particles is still relatively large relative to the dimensions required for certain applications such as luminescence. Moreover, monitoring the size of the particles is relatively difficult because the conditions for actually carrying out the process have a substantial influence on the particle size.